Seat back



FIG. 1 is a perspective view of a seat back comprising the new design;

FIG. 2 is a top view thereof;

FIG. 3 is a front elevational view thereof;

FIG. 4 is a right side elevational view thereof, where the left side elevational view is a mirror image thereof;

FIG. 5 is a rear elevational view thereof; and,

FIG. 6 is a perspective view of an alternate embodiment of a seat back, portions of which are cut away to illustrate indeterminate length, which is the sole difference between it and FIGS. 1 through 5.

The broken lines are for illustrative purposes and form no part of the claimed design. 

The ornamental design for a seat back, as shown and described. 